If you eat twice as much vitamin B12 from food, your blood levels of B12 don’t go up much—your body just doesn’t absorb it very well, especially as you get older.

When you eat fish, sheep meat, or chicken, your body absorbs vitamin B12 from them at different rates—fish gives you about 42% of the B12, sheep meat gives you 56% to 89%, and chicken gives you 61% to 66%, so not all meats give you the same amount of this important vitamin.

Your body can only absorb a little bit of vitamin B12 at a time—about as much as what’s in a single serving of meat or fish. If you eat or take way more than that in one meal, your body just can’t use the extra, so most of it goes to waste.

People with higher levels of vitamin B12 in their blood tend to have a slightly higher chance of dying from any cause — especially older adults — so high B12 might just be a sign that something else is wrong in the body.

People with vitamin B12 levels between 400 and 600 units in their blood are 34% more likely to die from any cause than people with lower levels—even though those levels are still considered normal by doctors.

In people with type 2 diabetes, having very high levels of vitamin B12 in the blood seems to be linked to a much higher chance of dying from heart disease—higher than at any other B12 level—so maybe high B12 isn’t helping, but instead signals something else wrong in the body.